• TI Thinks Resolved

UCC28070: Active power factor correction

Part Number: UCC28070

Hello,

I am using UCC28070 interleaved PFC controller for 1.2kW battery charger design. I have few queries regarding this IC. These are mentioned below.

1. Does duty cycle in boost PFC varies according to input sine voltage? 

2. What are the advantages and disadvantages if we keep duty cycle constant?

3.  What are the advantages and disadvantages if we keep duty cycle variable?

4. Why loop bandwidth of PFC is always kept low such as at 20Hz?

Regards,

Dhananjay

  • In reply to John Griffin1:

    Hello John,

    What is typical bandwidth of current loop and voltage loop?

    Regards,
    Dhananjay Yadav
  • In reply to John Griffin1:

     Hello John,

    Thanks for reply.
    Can you please let us know the use of the boost bypass diode as highlighted in attached image.
    Our input ac rms voltage range is 165 to 295 and o/p of PFC is 420V . So please let us know the use case of boos bypass diode in our application?

    Regards,
    Dhananjay Yadav

  • In reply to Dhananjay Yadav:

    Hello Can you please reply to above query?
  • In reply to Dhananjay Yadav:

    Hello Dhananjay

    The purpose of the bypass diode is to carry the initial inrush currents when the PSU is first connected to the line. The output capacitor will have 0V across it and will rapidly charge to the peak of line - this happens before the PFC controller starts. If this bypass diode is not fitted, then this current will flow through the PFC inductor and PFC diode but the problem is that the Fast diodes used for the PFC output diode don't normally have a high enough surge current rating - the parallel bypass diode keeps theese surge currents away from the PFC diode. It has no other function and is inactive during normal operation of the PFC stage. You can evaluate the surge currents in your own design and remove the bypass diode if your PFC diode can carry them - but be careful because this is not normally the case.

    Regards
    Colin
  • In reply to John Griffin1:

     Hello,

    1. Below screenshot is from datasheet of UCC28070 (Page no: 32). Please see highlighted box in red color. Can you please let us know why output capacitor is considered at twice the line frequency?

    2. Is Protection from undervoltage and overvoltage on AC input line is available on UCC28070?   

    Regards,
    Dhananjay

  • In reply to Dhananjay Yadav:

    Hello Dhananjay
    The answer to your first question is it will be minimum and to your second question it will be whatever maximum value is set on the DMAX pin.

    Regards
    John
  • In reply to John Griffin1:

    Sorry I was looking at an earlier question.
    The output ripple frequency is at twice the line voltage because of the bridge rectifier. This sets the low frequency output to twice the line.
    There is no protection from input overvoltage on the UCC28070.
    It does not have a brownout protection but at low line input the UCC28070 goes into current limit and the output voltage drops. Also the maximum duty cycle limit is reached which further tends to decrease VOUT.
    The UCC28070 turns off when VSENSE drops below its undervoltage limit

    Regards
    John
  • In reply to John Griffin1:

     Hello,

    1. Regarding low AC input voltage: It is understood that Vout will decrease as 28070 will go into current limit and duty cycle limit also reaches. But I could not find the value in datasheet at VSENSE pin where UCC28070 turns off. Please guide.

    2. If Under voltage protection is already present, then why it is added externally in TI's one of the reference design? Please refer to below image.(File: PMP4311A_Sch_RevA.sch)

    3. As there is no protection from over voltage , so we need to add external circuitry for this right?

    4. But in case of overvoltage, where VINAC peak value > VPFC output, 28070 will reduce duty cycle(28070 will go into voltage limit cycle) and at certain AC input over voltage it will become zero right?

    Regards,
    Dhananjay Yadav

  • In reply to Dhananjay Yadav:

    (1) Look in the data sheet for the section on "pfc enable".
    This value is 0.75V
    (2) Under voltage naturally occurs when the value of VSENSE <0.75 and then PWM ceases.
    If a brownout voltage needs to be set independently of VSENSE then an external circuit is needed.
    (3) This is totally dependent on your design requirements.
    (4) The UCC28070 stops PWM when the output voltage reaches 106% of the nominal set point

    John Griffin
  • In reply to John Griffin1:

      Hello John,

    Thanks.

    We have another query. In ZVT section, a resistor and capacitor is placed in series with gate drive transformer primary coil winding. Can you please let me know the reason behind this? And how to select these resistor capacitor values.

    When we are simulating below circuit, the waveform is settling after 300usec approx. Considerinf 200KHz switching frequency, will there be a problem? Please guide in selecting cap and resistor value.

    Regards,
    Dhananjay Yadav